Five analogues of iso-PhABA (20) developed earlier by our research group were designed and synthesized. The bioassay results show that the number and position of methyl groups along with the substitution of hydrogen atoms of the methyl group have a great influence on the activity. Compared with iso-PhABA, the inhibitory activity of diMe-PhABA (21) on seed germination and rice seedling growth decreased slightly; however, it significantly reduced the capability of inhibiting wheat embryo germination. Both 3′-deMe-iso-PhABA ( 22) and 2′-deMe-PhABA (23) exhibited weak inhibitory activities, and 11′methoxy iso-PhABA (24a/24b) was much more efficient than its isomer 24c/24d in all bioassays. These results reveal the preservation of quaternary carbon at the 2′ or 3′ position is necessary to maintain its ABA-like biological activity, and demethylation at the 3′ position has a more significant effect. The selectivity of these compounds to different physiological processes makes them available as selective probes for different ABA receptors.